Not applicable.
Not applicable.
The present invention relates to impurity monitors for gases, and, in particular, to a device for monitoring the total impurities in ultra high purity gases.
Ultra high purity gas is needed by, for example, the semiconductor fabrication industry where, according to current industry specifications, impurities typically may be required that are less than one part per billion. As the industry matures, these levels are expected to go even lower. At the present time, the practice in the industry is to certify compliance with these specifications at the time of commissioning of the gas delivery system that delivers these ultra high purity gases. Thereafter, real time upset monitors are used to detect large changes in the concentration of impurities. The main reason for not monitoring continuously, in real time compliance with the industry specifications, is the prohibitive cost of sophisticated analytical instruments needed for this purpose.
The objective of the present invention is to provide an inexpensive device that can monitor changes in the total impurity content in ultrahigh purity gases being supplied to, for example, the semiconductor industry.
At the present time, atmospheric pressure ionization mass spectrometry (APIMS) is the only practical analytical technique capable of achieving detection limits significantly below impurity levels of less than one part per billion. Attempts have been made to simplify this instrument so as to reduce its cost. For example, attempts have been made to devise a sample switching manifold so that one can use a single APIMS instrument and sample multiple sample points and/or gas streams. Efforts are also being made to improve the sensitivity of other analytical instruments so that they can be used for monitoring compliance with the above noted industry specifications where it is desired to have impurities at levels less than one part per billion.
One instrument is commercially available that can meet the one part per billion specification for trace oxygen in gases. This is the NanotraceJ O2 analyzer made by Delta F Corporation of Massachusetts. However, this device is sensitive to only oxygen gas, whereas an APIMS is sensitive to other impurities as well.
Unexamined Japanese Patent Kokai 9-6 1402 directed to a device for measuring concentration of impurities in gases describes the use of a discharging chamber to measure concentration of impurities in gases. A corona discharge is established between a needle and an electrode. A high voltage constant current source is used for the discharge so that the discharge current is maintained at a constant level. The sample gas flows through this discharge. The sample gas is supplied via a component separating means such as a chromatograph or a membrane. As the impurity concentration of the sample gas changes, the voltage needed to maintain a fixed current changes. The change in voltage is a measure of the impurity concentration.
Although the Unexamined Japanese Patent does not explicitly mention the use of this device to monitor the changes in the total impurity concentration of the sample gas, if one were to sample the gas without any component separating means, this device can be used to monitor changes in total impurity concentration.
A major problem with using a corona discharge device is that, over time, the tip of the discharge needle erodes, thereby changing the needle to electrode distance. This change in distance will cause a change in voltage needed to establish a constant current. Thus, the output of this device will slowly change over time without any changes in the impurity concentration in the sample gas.
Certain ion mobility spectrometers have a somewhat similar structure to that of the present invention. See, for example, U.S. Pat. No. 4,238,678, which includes a housing, an ionizing source such as Ni63, a shutter grid and a collector. However, in this type of spectrometer, the shutter is periodically opened and an ion cloud is allowed to enter the drift region of the spectrometer analyzer cell. The ion cloud moves in the drift region under the influence of an electric field. The ions are separated into different groupings, depending upon their mobilities. As each separated ion grouping arrives at the collector plate at the end of the drift region, an electrical pulse is detected by a detection circuit. A multichannel analyzer is typically used to average spectra from multiple openings of the electric shutter to produce an ion mobility spectrum. The present invention does not use a shutter that is periodically opened and closed.
It is principally desired to provide a novel device for measuring the total concentration of impurities in a sample gas.
It is further desired to provide a novel device for measuring the total concentration of impurities in a sample gas that can detect very low levels of impurities in the sample gas.
It is still further desired to provide a novel device for measuring the total concentration of impurities in a sample gas that can detect levels of impurities of less than one part per billion in the sample gas.
It is also desired to provide a novel device for measuring the total concentration of impurities in a sample gas continuously and in real time.
It is also desired to provide a novel device for measuring the total concentration of impurities in a sample gas continuously and relatively inexpensively.
It is further desired to provide a novel device for measuring the total concentration of impurities in a sample gas that yields a consistent response through the lifetime of the device.
It is further desired to provide a novel device for measuring the total concentration of impurities in a sample gas that does not require a high voltage power supply to generate electrons.
It is further desired to provide a novel device for measuring the total concentration of impurities in a sample gas that is simple and inexpensive.
The present invention is directed to a device for measuring a total concentration of impurities in a sample gas which includes a housing having an inlet to allow the sample gas to enter the housing, an emitter to generate ions from the sample gas, and a field gradient to accelerate the ions toward a collector. The collector measures total ions, and an outlet on the housing allows the sample gas to exit the device. A change in total ions measured indicates a change in a total concentration of impurities in the sample gas. Preferably, the emitter is a radioactive foil of Ni63 that emits approximately 67 keV electrons and has a strength of 1 milli Curie. Additionally, it is preferable that the collector is connected to an amplifier which is used to detect current striking the collector. It is also desirable that a ground base preamplifier is connected between the collector and the amplifier such that the voltage at the collector is at zero volts. Optionally, at least one grid electrode is located in the housing between the emitter and the collector to facilitate ions in moving from the Ni63 xcex2 emitter to the collector. Alternatively, the inside of the housing includes a resistive coating to facilitate ions in moving from the emitter to the collector. The housing is preferably fabricated from metal, such as electro polished stainless steel that can be heated to at least 200 degrees Celsius.